1. Field of the Invention
The present invention relates to an image reading apparatus, and more particularly to an image reading apparatus which reads an original using either a sheet-through-type or book-type original reading system by performing shading correction based on a white reference plate, and which increases the processing speed by adjusting the timing of various operations when reading is performed using the sheet-through system.
2. Description of the Background Art
In an image reading apparatus such as a copying device or scanner, a so-called book system and a so-called sheet-through system are used as systems for reading an original image. In a book system image reading apparatus, an original placed on a contact glass is held in place by a platen, and a white reference plate provided on the surface of the platen on the contact glass side is read in order to correct the gain of a signal processing unit and generate shading data. An image of the original is then read while moving an image reading portion carried on a moving traveling body over the original, and thus high-precision image reading is performed. In a sheet-through system image reading apparatus, an original image is read by moving the original while a fixed exposure scanning unit does not move.
In an image reading apparatus which is capable of performing reading using both the book system and the sheet-through system, a so-called sheet-through-type document feeder (automatic original conveyance mechanism) comprising an original placing portion on which a plurality of originals is placed, a conveying portion for conveying the originals on the original placing portion sheet by sheet, and a sheet-through contact glass provided between the conveyed original and the exposure scanning unit is provided so that a plurality of originals can be read efficiently. When reading is performed using the sheet-through system, to ensure that the correction amount is equal to that of the book system, the white reference plate is read by moving the exposure scanning unit carried on the traveling body between the white reference plate and the sheet-through contact glass, thus correcting the gain of the signal processing unit and generating shading data, whereupon the exposure scanning unit is moved to the position of the sheet-through contact glass, and an image of the original conveyed by the document feeder is read by the exposure scanning unit through the sheet-through contact glass. Thus image reading is performed with high precision.
Further, in the case of the sheet-through system, the original comes into contact with the sheet-through contact glass, and hence the sheet-through contact glass may become dirty. This dirt may appear on the read image as a line in the reading direction at the soiled part. Hence, in order to reduce the occurrence of lined images caused by dirt on the sheet-through contact glass in conventional sheet-through system image reading apparatuses, several lines of a dirt reference plate positioned on the upper face of the sheet-through contact glass are read following shading processing, and when dirt is detected from the image data obtained when this dirt reference plate is read, correction processing is typically performed.
However, in an image reading device that is capable of reading using both the sheet-through system and book system, when reading is performed using the sheet-through system by conveying a plurality of originals through a document feeder sheet by sheet, a process whereby the traveling body loaded with the exposure scanning unit is moved to the position of the white reference plate to read the white reference plate, and then returned to the sheet-through contact glass position, which is the original reading position, to read several lines of the dirt reference plate in order to detect dirt must be performed every time an original is read. As a result, the processing routine cannot advance to reading of the next original until shading correction and dirt detection are complete, leading to a corresponding deterioration in the image reading productivity (number of originals read per hour).
To solve this problem, it is possible to reduce the time required for shading by increasing the speed at which the traveling body loaded with the exposure scanning unit moves to the white reference plate and returns to the reading position, but if this speed is increased excessively, positional deviation and so on of the exposure scanning unit become more likely to occur, and hence there are limitations thereon as a mechanical operation.
Hence a conventional image processing apparatus has been proposed in Japanese Unexamined Patent Application H6-284284. This image processing apparatus comprises: reading means for reading an original image in line units, these means having a light source and a one-line light-receiving element; moving means for moving the reading means from a white reference set position, at which a white reference value for shading correction is set, to an original reading position at which the original image is read; a main white reference plate facing all of the light-receiving elements of the reading means set in the white reference set position; a sub white reference plate facing an end portion light-receiving element of the reading means set in the original reading position; original conveying means for conveying a plurality of originals in succession such that the remaining light-receiving elements of the reading means set in the original reading position read the original image; and processing means for retrieving from the main white reference plate in the white reference set position an amount of light entering each light-receiving element of the reading means as a white reference value, retrieving from the sub white reference plate in the reading position an amount of light entering the end portion light-receiving element of the reading means every time an original image is read, and modifying each white reference value of the light-receiving elements in accordance with the ratio of the light amount entering the end portion light-receiving element in the white reference set position and the original reading position.
Further, a conventional image reading apparatus has been proposed in Japanese Unexamined Patent Application H8-9116. This image reading apparatus has a first original reading mode in which an original image is read by having an optical system scan an original placed on an original table, and a second original reading mode in which the original image is read with the optical system stopped in a specific position while the original is conveyed by original conveying means, original images read in both modes being subjected to photoelectric conversion from an optical signal to an electric signal using a photoelectric conversion element. The image reading apparatus comprises a first white reference plate serving as a white reference of the first original reading mode, and a second white reference patch serving as a white reference of the second original reading mode. Shading correction in the first original reading mode and shading correction in the second original reading mode are both performed on the basis of shading waveforms obtained by reading a common white reference plate.
These prior art examples both perform correction on the basis of values corrected by a white reference plate, using a sub reference plate each time reading is performed.
A further image reading apparatus has been proposed in Japanese Unexamined Patent Application 2001-53938. This image reading apparatus comprises an exposure optical system for reading an original that is conveyed over a platen at a fixed speed, a shading plate for correcting the white reference of the read image information, a traveling body for moving the reading position of the optical system in order to read the shading plate using the exposure optical system, and control means for performing a shading operation by moving the traveling body to the position of the shading plate both before and after the original is read.
In this type of conventional image reading apparatus, however, improvements are required in the efficiency and quality with which images are read. More specifically, in the prior art of Japanese Unexamined Patent Application H6-284284 described above, the sub reference plate is provided in only one part outside of the reading range, and hence irregularities in the main scanning direction cannot be corrected accurately. In the prior art of Japanese Unexamined Patent Application H8-9116 described above, irregularities in the main scanning direction can be corrected, but since a mirror must be driven, a mechanical operation in which the mirror is moved to the direction in which the sub reference plate is read and then returned to its state during original reading is required, and hence there are limitations on the extent to which the processing time can be shortened.
In the prior art of both Japanese Unexamined Patent Application H6-284284 and Japanese Unexamined Patent Application H8-9116, two white reference plates are provided, and as a result of this increase in the number of white reference plates to be read, deviations occur from the original correction values.
In the prior art in Japanese Unexamined Patent Application 2001-53938, a shading operation is taken into consideration, but the time required for the original to move from the standby position to the reading position is not considered, and hence it may be impossible to achieve the intended effects due to the reading speed of the document feeder. Hence improvements are needed.